1. Field of the Invention
This invention relates to a light diffusive coating which is formed on a base surface of lamps such as halogen lamps, high pressure discharge lamp bulbs, optical filters, window glass, etc., giving a diffusive characteristic, and to a method of forming usch a coating, and which makes possible lamps which are more efficient and give uniform illumination.
2. Description of the Prior Art
In order to give uniform distribution of illumination over the illuminated surface, halogen lamps for photocopiers, for example, have several filaments connected in series through conductors, and the whole assembly is enclosed in a straight tube type vessel made of silica glass so that it lies along the axis of the tube. However, because this halogen lamp includes a large amount of infra-red radiation in its emissions, there is a danger that the item being photocopied will be damaged by the heat. For this reason, the lamp surface generally undergoes a honing treatment to provide a diffusive surface. This method, however, has the disadvantage of being very laborious. In addition, the methods of forming a light diffusive coating by applying a fine diffusive powder such as silica to the outside of the lamp by electrostatic coating and etching may be considered. These types of coating are generally weak for mechanical stresses, wear easily, and in some cases the light diffusive effect falls off with a long period of use. Dependent on the type of fine light diffusive powder, there are cases where the coating has a poor affinity for the quartz glass, and readily peels off.
With regard to these points, the applicants have previously developed the technology of forming a coating on the surface of the transparent outer vessel which allows visible light to pass through, but reflects infra-red light, and by providing a light diffusive coating on top of the infra-red reflective coating, the visible light which has passed through the infra-red reflective coating is dispersed by the light diffusive coating, giving a uniform distribution of illumination on the illuminated surface. This was proposed in the Japanese Patent Application Number Sho 58-95001/1983). However, when this technology is applied to lamps, such as halogen lamps for example, because the temperature of the lamp's surface is extremely high, with some light diffusive coatings, there is the danger that the light diffusive coating may peel off due to the effect of repeated switching on and off over a long period. This means that it is essential to select a light diffusive coating which is stable regardless of the rise and fall in temperature and which is mechanically strong.